H-plane t-junction comprising at least two separable waveguide sections



NOV. 24, 1970 J WHITE 3,543,199

H-PLANE T-JUNC N COMPR NG L T TWO SEPARAB WAVEGUI SE ON Original Filed April 28, i967 2 Sheets-Sheet 1 NOW 1970 J. R. WHITE H-PLANE T-JUNCTION COMPRISING AT LEAST TWO SEPARABLE WAVEGUIDE SECTIONS Original Filed April 28, 196'? 2 Sheets-Sheet 2 A I I I I INVENTOR.

JEROME R. WHITE United States Patent 3,543,190 H-PLANE T-JUNCTION COMPRISING AT LEAST TWO SEPARABLE WAVEGUIDE SECTIONS Jerome R. White, San Carlos, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Original application Apr. 28, 1967, Ser. No. 634,522. Divided and this application Feb. 26, 1969, Ser.

Int. Cl. H01p 5/12 US. Cl. 3339 3 Claims ABSTRACT OF THE DISCLOSURE The slotted waveguide applicator includes two sets of open-ended U-shaped half waveguides with the half waveguides of each set parallelly aligned. The sets of half waveguides are mounted spaced apart to define contiguous slotted wide walls. Notches are provided in the contiguous arms of each half waveguide to define coupling holes inwardly displaced from opposite ends of the waveguides. An H-plane T-junction microwave guiding structure formed from L-shaped half sections, with each half section including one half of the side arm and a length of the main transmission line, couples a microwave source connected to one end of the main transmission line to the applicator connected to the side arm. An inductive window is provided at the junction and a shorting plate terminates the remaining end of the main transmission line. A second *H-plane T-junction microwave guiding structure connects a dummy load to the end of the applicator.

CROSS-REFERENCE TO RELATED APPLICATION This is a divisional application of copending application Ser. No. 634,522, filed Apr. 28, 1967, now Pat. No. 3,471,- 672, dated Oct. 7, 1969.

BACKGROUND OF THE INVENTION The present invention relates generally to slotted waveguide microwave heating devices, and more particularly, to a serpentine type microwave heating device.

Slotted waveguide type microwave heating devices of applicators commonly are employed to heat a workpiece under continuous fiow feed conditions. In most applications, a plurality of slotted waveguide sections are joined to define a serpentine path for electromagnetic wave energy for heating the workpiece as it is passed through each waveguide section. In such applicators, the workpiece to be heated is passed through the waveguide section with its surfaces parallel to the electric field component of the electromagnetic field established therein. Contrary to most other types of applicators, slotted waveguide type applicators can be excited to present a uniform electromagnetic field distribution to workpieces substantially larger than the wavelength of the applied energy.

Many uses of serpentine applicators require that they be cleaned often. Since the interior of such applicators are closed to the surroundings, it is necessary to disassemble them in order to clean the interior or undertake most other preventative maintenance. Furthermore, the structure of many applicators results in the disassembly and assembly tasks being time-consuming and costly. In this connection, it is generally necessary to uncouple the applicator from its microwave power source before the applicator can otherwise be disassembled.

SUMMARY OF THE INVENTION A unique technique of intercoupling the waveguide sections of an applicator facilitates constructing a compact "ice applicator which easily can be manufactured and repeatedly assembled and disassembled. These features are realized by forming the waveguide sections from first and second sets of identical half sections of waveguides arranged in a parallel array with adjacent walls contiguous and with the half sections of each set fixed together and demountable from those of the other set as a unit. In accordance with the invention, a novel H-plane T-junction structure is provided which permits the power source for the applicator to be mechanically coupled to only one of the two half sections of the applicator so that it is unnecessary to uncouple the source from the applicator be fore the half sections are separable. The junction comprises a main transmission line portion and a side arm line portion joined to define the junction, a short circuit provided at one end of the main transmission line portion and an inductive means mounted in the main transmission line and side arm portions. The side arm of the junction is coupled to the applicator and the power source is connected to the end of the transmission arm which is not shorted. The effective reactance of the inductive window and the distance between the short circuit means and the junction is adjusted to provide maximum transmission of power between the side arm and the end of the main transmission line to which the power source is connected. Desirably, the T-junction is made up of at least two separable waveguide sections which meet at a joint extending along the side arm portion. If each of the separable waveguide sections of the T-junction is associated with a corresponding one of the half sections of the applicator, the result is that the half sections of the applicator can be separated without the necessity of disconnecting the power source from the T-junction.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects and advantages of the microwave applicator of the present invention will become more apparent from the following detailed description and appended claims considered together with the accompanymg drawings in which:

FIG. 1 is a perspective view of a microwave applicator according to the present invention.

FIG. 2 is a cross sectional view of the applicator taken along lines 22 of FIG. 1.

FIG. 3 is an exploded perspective view of a portion of the applicator of FIG. 1.

FIG. 4 is a cross sectional view of the T-junction taken along lines 44 of FIG. 1.

FIG. 5 is a partially broken perspective view portraymg an alternate way of mounting the T-junction of FIG. 4 to a waveguide.

With reference to FIGS. 1-2, the slotted waveguide applicator 10 includes a plurality of waveguides 11 having apertures such as slots 12 for passing a workpiece through the waveguide. The waveguide 11 are intercoupled in series through coupling holes 13 to define a serpentine path for electromagnetic energy provided by source 14. To minimize power losses through radiation, the width W of the slot apertures 12 should be no greater than is necessary to allow easy transport of the workpiece, and preferably, no greater than one-third the distance between the slot apertures of one waveguide 11.

A uniform electric field intensity is desired in the vicinity of the workpiece so that it may be uniformly heated. Such a field can be realized by, for example, employing rectangular waveguides 11 with slots 12 provided in opposite wide walls 16 and 17 which is excited to propagate TE waves having an electric field field component normal to the wide walls. In accordance with standard practice, slots 12 are located in the center of the wide walls 16 and 17 to minimize leakage and to maximize the energy coupled to the workpiece.

A unique H-plane T-junction waveguide structure 18, shown in detail in FIG. 4, is employed to couple the applicator 10 to a waveguide transmission line 19 from source 14. As will be explained in greater detail hereinbelow, the unique H-plane T-junction 18 of FIG. 4 can provide essentially reflectionless power transfer from the waveguide 19 to the waveguide 11 of the plurality of intercoupled waveguides.

The slotted waveguide applicator 10 can be operated either as a standing-wave or as a traveling-wave applicator, the mode of operation depending upon how the applicator is terminated. In either operating mode, the required termination should be constructed to operate with the applicator 10 energized in the presence of a workpiece. By terminating the end waveguide or waveguides 11 of the intercoupled waveguides in a reflectionless dissipative load 21, the applicator 10 is operated as a transmission line type traveling-wave applicator. T he load 21. can be coupled to the applicator 10 by another H-plane T-junction waveguide structures 18' constructed like the H-plane T-junction 18.

The coupling holes 13 are located inwardly from the ends 22 and 23 of the waveguides 11. To control coupling between successive waveguides 11, conductive members 24 and 26 are positioned respectively in the waveguide sections 27 and 28 of waveguides 11 defined between coupling holes 13 and each of the waveguide ends 22 and 23. The conductive members 24 and 26 are constructed and positioned in the waveguide sections 27 and 28 to define a short circuit path for the electric field component of the electromagnetic field propagated through the waveguide sections 11. To minimize reflections at the coupling holes 13 intercoupling successive waveguides 11, each of the conductive members 24 or 26 in the successive waveguides is constructed and positioned relative to its associated coupling hole 13 for minimum power reflection. The position for such a condition can be determined by standard empirical power reflection measuring techniques.

The structure of this waveguide intercoupling arrangement enables each waveguide 11 of the applicator to be open to the surroundings thereby providing easy access to any part of the interior of the applicator, even during operation. More particularly, by constructing the waveguide sections 27 and 28 and conductive members 24 and 26 and locating the conductive members 24 and 26 in the waveguide sections so that the sections are waveguides beyond cutoff at the wavelength of the applied energy, electromagnetic energy at a wavelength equal to or greater than that of the applied energy will not be freely trans mitted bythe waveguide sections 27 and 28. Hence, the ends 22 and 23 of waveguides 11 can be opened to the surroundings of applicator 10 without radiating hazardous or undesirable electromagnetic energy to its surroundings unless higher order modes are present in the excited waveguides 11. However, by proper choice of the dimensions of the waveguides 11 with respect to the frequency of the applied energy, free transmission of the dominant mode can be allowed through the waveguides 11 while that of the higher order modes is impeded. If the higher order modes also are prevented from being freely transmitted through the waveguide sections 27 and 28, the applicator 10 will not radiate any significant electromagnetic energy to its surrounding through its open waveguide ends 22 and 23. Effective suppression of hazardous and undesirable electromagnetic energy radiation can be realized by constructing the waveguides 11 to have a cross sectional dimension so that the free space cutoff wavelength of the dominant mode is greater than the free space operating wavelength and the free space cutoff wavelength of the next higher mode is less than the free space wavelength of the operating mode. With the cross sectional dimensions of the waveguides 11 adjusted in accordance with these limitations and the applicator 10 excited with electromagnetic energy at a frequency higher than the cutoff frequency of the dominant mode but lower 4 than the cutoff frequency of the next higher mode, the waveguide 11 will not support the free transmission of higher order modes.

To circulate air through the applicator 10, the open ends 23 of the waveguides 11 along one side of the applicator could be inserted into an inlet plenum 29 suitably coupled to one or more air pumps 31 as the case may require. To convey away the circulated air, the open ends 22 of the waveguides 11 along the other side of the applicator 10 could be inserted into an exhaust duct 32 coupled to exhaust ports 33.

Towards ease of manufacturing, ease of repeated assembling and disassembling, and ease of maintenance, it is contemplated that a preferred embodiment of the applicator 10 will be constructed from first and second sets 34 and 35 of identical half sections of parallel waveguides arranged with adjacent walls contiguous. The half sections of waveguides of each of the sets 34 and 36 are fixed together and demountable as a unit from those of the other set.

Referring now to FIGS. 1-3 in detail, the particular preferred embodiment of the applicator 10 includes a first set '34 of rectangular U-shaped open-ended half waveguides 47 parallelly disposed with adjacent arms 48 of adjacent half waveguides 47 contiguous. The half waveguides 47 are secured in place by fastening them to two parallelly extending U-shaped channels 49, for example, with screws 51 threadingly engaging the half waveguide web portions 52. Each of the contiguous arms 48 of the half waveguides 47 define a rectangular U-shaped notch 53 inwardly spaced from the end of the half waveguides, with the notches 53 in contiguous arms 48 registered. The notches 53 of successive contiguous arms 48 are located inwardly from opposite ends of the half waveguides 47 whereby the notches of alternate contiguous arms are registered.

The applicator 10 is completed by mounting an identical second set 36 of rectangular U-shaped open-ended half waveguides 56 spaced below the first set 34 to define the slots 12 for passing a workpiece through the applicator. The half waveguides 56 of the second set 36 are secured in place to two U-shaped channels 57 'by, for example, screws 58 to coextend with the half waveguides 47 of the first set 34. The arms 59 of the half waveguides 56 define rectangular U-shaped notches 61 which are aligned over the notches 53 of the half waveguides 47 of the first set 34 when the applicator 10 is assembled.

To maintain the desired coupling between all of the waveguides 11 and prevent the excape of undesirable electromagnetic energy from the open ends 22 and 23 of the Waveguides 11, each of the half waveguides 47 and 56 of the waveguides should be in good electrical connection with the conductive members 24 and 26. To insure that good electrical connection is maintained between the conductive members 24 and 26 and the half waveguides 47 and 56, each of the conductive members 24 and 25 is formed from a pair of thin conductive strips, 37 and 38 and 39 and 40 respectively, longitudinally extending the length of the applicator 10. One of each pair of strips, e.g., 37 and 39, are fastened by nuts 64 and bolts 66 to each of the half waveguides 47 of the first set 34, and the other strips '38 and 40 of each pair are fastened by nuts 64 and bolts 66 to each of the half waveguides 56 of the second set 36. The conductive strips are fastened by the nuts 64 and bolts 66 to press firmly against the half wavgeuide arms 48 and 59 and thereby form good electrical contacts therebetween.

When the applicator 10 is assembled, the half waveguides 47 and 56 define the plurality of open-ended slotted waveguides 11 with the end waveguide sections 27 and 28 of each waveguide 11 and conductive strips 37-40 forming waveguide sections 27 and 28 beyond cutoff. It should be noted that in those cases where the first and second sets 34 and 36 of half waveguides 47 and 56 are spaced so that the pairs of conductive strips forming conductive mem.

hers 24 and 26 do not touch, the portion of the half waveguides 47 and 56 defining the end waveguide sections 27 and 28 and the conductive strips 37-40 will form at each end of the waveguides 11 two spaced apart waveguides beyond cutoff and the space therebetween will form an extension of the non-radiating slot 12.

The U-shaped notches 53 and 61 of the assembled half waveguides form a rectangular microwave coupling hole 13 in the contiguous walls of adjacent waveguides 11.

For practical considerations, it is preferred to couple electromagnetic energy to the applicator by a waveguide type microwave guiding structure 18. In order to be able to disassemble the applicator 10 Without having to disconnect the electromagnetic energy source 14 therefrom, the'unique H-place T-junction structure 18, shown in detail in FIG. 4, is employed to couple energy to the applicator 10. More specifically, in one embodiment, the H-plane T-junction structure 18 is formed of demountable L-shaped half sections 72 and 73. The side arm 74 of the T-junction 18 is defined by half waveguide extensions 76 and 77 of the half waveguides 47 and 56 of the first waveguide 11 of the serially coupled waveguides forming the applicator 10. The half waveguide extensions 76 and 77 extend from the end of the first waveguide 11 distal the rectangular coupling hole 13. The waveguide extensions 76 and 77 are integrally joined respectively to flanged waveguide sections 78 and 79 which when assembled form the main transmssiion line portion 80 of the H-plane T- junction 18.

In operation, the electromagnetic energy source 14 is coupled, for example, to the flanged waveguide section 78 by the waveguide transmission line 19 connected to the flange 81. The other flanged waveguide section 79 is terminated with short-circuit termination 82 joined to the flange 83. The short-circuit termination 82 cooperates with an inductive window 84 located at the junction 86 of the main transmission line 80 and side arm 74 to cause all of the energy supplied by the source 14 to be coupled into the side arm 74, hence, the applicator 10. As in the case of the conductive members 24 and 26, inductive window 84 can include two plates 87 and 88 each fastened at opposing locations of the flanged waveguide sections 78 and 79 to provide good electrical connection between the inductive window 84 and sections of the main transmission line 80. The resulting symmetrical arrangement of the window about the joint between the sections 78 and 79 assures that the window does not cause radiation leakage at the joint.

In order to excite the waveguide 11 from one end as described immediately above, the pair of conductive strips 39 and 40 are positioned so as not to extend between the half waveguides 47 and 54 defining the waveguide 11 to which the source 14 is coupled.

To protect against the generation of damaging reflections and to dissipate unabsorbed power transmitted through the applicator 10, the open end of the last waveguide 11 of the structure distal its rectangular coupling hole 13 is joined to the H-plane T-junction microwave feed 18' which is identical to the H-plane T-junction 18. One end of its main transmission line 89 is joined to a standard water type dissipative load 21. The other end (not shown) of the main transmission line 89 is shorted to cooperate with an inductive window (not shown) to prevent reflections in the same manner as the corresponding elements of H-plane T- junction 18. Furthermore, the pairs of conductive strips 39 and 40 are positioned so as not to extend between the half waveguides 47 and 54 defining the waveguide section 11 to which the water load 21 is coupled.

With particular reference to FIG. 5, an alternate arrangement for coupling microwave energy to a waveguide 91 with the unique H-plane T- junction structure 18 is shown. In this embodiment, the main transmission line portion 92 of the T-junction 18 is coupled at one end 93 in line with the waveguide 91 with the side arm 94 coupled to the source (not shown). The other end 96 of the main transmission line 92 is provided with a conductive member 97 positioned in the main transmission line 92 to define a short circuit path for the electric field component of the electromagnetic field propagated into the waveguide 91. An inductive window 98 is located opposite the side arm 94 and cooperates with the short circuit forming conductive member 97 to provide essentially reflectionless power transfer from the side arm 94 to the waveguide 91. This arrangement for coupling electromagnetic energy to a waveguide with the unique H-plane T-junction 18 of FIG. 4 would be preferred when it is desirable to circulate a gaseous medium through all parts of a waveguide applicator. To establish an air flow through one slotted waveguide without radiating undesirable electromagnetic energy to its surroundings, the end 99 of the waveguide 91 distal T-junction 18 would be opened and provided with a second short circuit forming conductive member 101.

Referring again to FIGS. 1 and 2, the inlet plenum 29 for circulating air or other gaseous medium through the waveguides 11 of applicator 10 is a rectangular enclosure having an open side 106 mounted by brackets 107 to the U-shaped channels 49. The open side 106 receives the open ends 23 defined by the assembled half waveguides 47 and 54. Brackets 107 also serve to support the air pumps 31. Air is delivered to the plenum 29 through inlet ports 108 coupled to the air pumps 31 by conduits 109. A rectangular exhaust duct 32 is secured by brackets 111 to the U- shaped channel 49 to convey away the circulated air. The exhaust duct 32 has an open side 112 to receive the open ends 22 defined by the assembled half waveguides 47 and 54 and collect the circulated air therefrom for exhausting through the exhaust ports 33.

Referring to FIG. 4, the unique H-plane T-junction structure 18 which is able to couple all the power entering the junction 86 between one end of the main transmission line and the side arm 74 is illustrated in detail. As noted hereinbefore, when such a T-junction is employed to couple power into and terminate the microwave heating device of the present invention, assembly and disassembly of the device is greatly simplified. Furthermore, such a microwave guiding structure can greatly facilitate the mass production of the microwave heating structure since both sets 34 and 36 of half waveguides will be identical from input to ouput. More specifically, the total power transfer between the waveguide section 78 of the main transmission line 80 and the side arm 74 is accomplished by selecting an inductive window 84 of proper width, W and by positioning the shorting plate 82 the proper distance from the junction 86. The width, W determines the degree of coupling between the side arm 74 and main transmission line 80, and the distance between the shorting plate 82 and junction 86 determines the frequency at which power is transferred. With minimum reflection with a proper choice of the width of the inductive window 84 and of the distance that the shorting plate is located from junction 86, the normalized impedance of the T-junction 43 will equal 1+10. In this condition the voltage standing wave ratio (VSWR) will be unity. Hence, for a given microwave frequency and given size H-plane T-junction 18, the proper inductive window 84 and location of shorting plate 82 can be determined by monitoring the VSWR as the width, W window 84 and location of shorting plate 82 is varied. When the VSWR approaches unity, the reflected power approaches zero. The width, W and the location of the shorting plate 82 is adjusted until the VSWR indicates that power is transferred between the waveguide section 78 and side arm 74 with an insignificant amount of reflection. In the aforedescribed specific embodiment, the width, W of the inductive window 84 was 1.0 inch, and the shorting plate 82 was located 7% inches from the junction 86.

Although an asymmetrical type inductive window structure is illustrated in the figures, with a proper choice of the window size and location of the shorting plate 82 relative to junction 86, it is possible to employ symmetrical inductive windows or combinations of capacitive and in- 7 ductive windows to form an H-plane T-junction 18 in accordance with the present invention.

While the present invention has been described in detail with respect to particular embodiments, it will be apparent that numerous modifications and variations are possible within the spirit and scope of the invention. Hence, the present invention is not intended to be limited except by the terms of the following claims.

What is claimed is:

1. Electromagnetic waveguide apparatus comprising a main transmission line portion and a side-arm transmission line portion jointed to define an H-plane T-junction, a short circuit means provided at one end of said main transmission line portion, and inductive means mounted in said main transmission line portion at the junction, the effectice reactance of said inductive means and the distance between said short circuit means and said junction adjusted to provide maximum transmission of power between the remaining end of said main transmission line and said side arm, said T-junction comprising at least two sep- 8 arable waveguide sections meeting at a joint which extends generally along the midplane of said side-arm portion.

2. The electromagnetic Waveguide apparatus of claim 1 wherein said inductive means is symmetrical with respect to said joint between said two separable waveguide sections.

3. The electromagnetic waveguide apparatus of claim 1 wherein said inductive means mounted in said main transmission line portion at the junction is an inductive window.

References Cited UNITED STATES PATENTS PAUL L. GENSLER, Primary Examiner US. Cl. X.R. 

